SEOUL — Any nuclear attack in Korea Peninsula would trigger an "effective and overwhelming" response, US Defense Secretary James Mattis said in Seoul on Friday as he sought to reassure Washington’s Asian allies following President Donald Trump’s inauguration.

Mattis was in the South Korean capital before going on to Tokyo, on the first overseas tour by a senior Trump administration official as concerns rise about the direction of US policy in the region under the protectionist and fiery leader.

South Korea has enjoyed US security protection since the 1950-53 Korean War, but on the campaign trail, Trump threatened to withdraw US forces from it and Japan if they do not step up their financial support.

Some 28,500 US troops are based in South Korea and 47,000 in Japan.

"Any attack on the United States or our allies will be defeated and any use of nuclear weapons would be met with a response that would be effective and overwhelming," Mattis told reporters ahead of a meeting with his South Korean counterpart Han Min-Koo.

He was in Seoul to "underscore America’s priority commitment to our bilateral alliance" and make clear the administration’s "full commitment" to defending South Korea’s democracy," he said.

Han added that the alliance "reaffirms its firm will and strength to remain unwavering against all challenges and adversaries".

North Korea carried out two atomic tests and a series of missile launches last year, and casts a heavy security shadow over the region.

Leader Kim Jong-un said in his closely-watched New Year speech that Pyongyang was in the "final stages" of developing an intercontinental ballistic missile, prompting Trump to tweet: "It won’t happen!"

’Top priority’

On Thursday Mattis and South Korean Prime Minister Hwang Kyo-ahn agreed to push through with the deployment of a US missile defence system strongly opposed by China.

The two confirmed that they will go ahead with the installation of the Terminal High Altitude Area Defense (THAAD) system in the South this year as planned.

Mattis’ visits to South Korea and Japan, he added, were "a message that the Trump administration is giving top priority to ensuring security on the Korean peninsula and the US is a reliable security partner in the region".

Japanese Prime Minister Shinzo Abe - who is scheduled to meet Trump next week in Washington - told lawmakers he intends to press Mattis about "the significance of the Japan-US alliance".

Mattis’ tour comes as relations between the US and other world powers such as Mexico and Australia get off to a rocky start.

The Washington Post reported late on Wednesday that Trump ripped into his Australian counterpart Malcolm Turnbull during a call last week, with the US president apparently fuming at a refugee accord he called "dumb" and cutting the conversation short.

Australia is a close US ally, and one of the so-called "Five Eyes" countries with which the US routinely shares sensitive intelligence.

Trump has meanwhile angered Mexicans by ordering the construction of a massive border wall and vowing to make their country pay for it.— AFP